This invention relates generally to means enabling improved light output from a metal halide discharge lamp and more particularly to achieving the improvement with heat sink means which regulate mercury condensation within the lamp arc tube in a distinctive manner.
Various metal halide discharge lamps commonly employ a fused quartz arc tube as the light source by reason of the refractory nature and optical transparency of this ceramic material. In such type lamps the arc tube generally comprises a sealed envelope formed with fused quartz tubing with discharge electrodes being hermetically sealed therein. A typical arc tube construction hermetically seals a pair of discharge electrodes at opposite ends of the sealed envelope although it is also known to have both electrodes being sealed at the same end of the arc tube. The sealed arc tube further contains a fill of various metal substances which become vaporized during the discharge operation. The fill includes mercury, sodium and metal halides along with one or more inert gases such as krypton, argon and xenon. Operation of such metal vapor discharge lamps can be carried out with various already known lamp ballast circuits employing either alternating current or direct current power sources.
For rapid sustained illumination with metal halide lamps, such as a xenon-metal halide lamp, a performance requirement now exists for at least fifty percent of the steady state light output to be reached within 0.75 seconds from the moment of lamp start-up. The prior art lamps experience significant light loss during start-up when the xenon discharge illumination is either absorbed or scattered by mercury which condenses upon the arc tube walls when first vaporized from the discharge electrodes. A "light hole" thereby results between the xenon illumination and less rapid illumination being produced by vaporization and ionization of the mercury and other metal ingredients further contained in the arc tube. By minimizing the light hole in these prior art lamps, a more sustained or continuous source of illumination is thereby provided.
Improved discharge electrode means are disclosed in the above referenced application to minimize occurrence of a light hole during lamp start and restart. A particular combination of anode and cathode means is therein disclosed significantly reducing the rate and maximum accumulation of mercury condensate on the arc tube walls during lamp start-up. Such improvement results from thermally managing mercury condensation during lamp cool-down so that condensation takes place adjacent the electrode means. More mercury is caused to condense at the anode end of the arc tube than condenses at the cathode end of the arc tube. Subsequent vaporization of condensed mercury from the anode means is also retarded during lamp restart. The cathode means have a dissimilar structural configuration relative to the anode means so as to exhibit a more rapid heating rate than the anode means during lamp start-up while further exhibiting a less rapid cooling rate than the anode means during lamp cool-down.
It is desired that further efficiency promoting means be provided to reduce light loss in a metal halide lamp. It is also desired to provide a metal halide lamp wherein condensed mercury is ionized without significant redeposition on the arc tube walls. In addition, it is desired to provide means enabling more rapid ionization of condensed mercury in a metal halide lamp.
Accordingly, it is an object of the present invention to provide means whereby metal halide lamps experience less light loss during start-up.
Another object of the present invention to provide an improved metal halide lamp employing a fused quartz arc tube as the light source which includes means for improved control of mercury condensation on the arc tube walls.
It is a still further object of the present invention to provide an improved automotive headlamp employing a metal halide lamp as the light source which experiences less light loss during start-up.
These and other objects of the present invention will become apparent upon considering the following more detailed description.